1. Field of the Invention
The present invention relates to a transmitting/receiving apparatus and a method thereof. More particularly, the present invention relates to a transmitting/receiving apparatus and a method for performing communication using a codebook.
2. Description of the Related Art
Recently, as the demand for high speed and high quality data transmission increases, a Multiple Input Multiple Output (MIMO) technique, which uses a plurality of transmission/reception antennas, is drawing attention as a technique for meeting this demand. The MIMO technique may improve a channel capacity as compared to a single antenna system by performing communication using a plurality of channels via a plurality of antennas. For example, when both the transmission and reception ends use M transmission antennas and reception antennas, a channel of each antenna is independent, and a bandwidth and entire transmission power are fixed, an average channel capacity increases by M multiples compared to a single antenna.
The MIMO technique may be classified into Open-Loop MIMO (OL-MIMO) and Closed-Loop MIMO (CL-MIMO) depending on whether channel information is fed back. In the CL-MIMO, a receiver feeds back channel information to a transmitter. The CL-MIMO may be further classified into two kinds. One kind of CL-MIMO allows a receiver to quantize a channel coefficient value and feeds back the coefficient value to a transmitter. In this method, although the transmitter may recognize a more accurate value regarding a channel, a feedback overhead occurs. The other kind of CL-MIMO allows a receiver to search for a codebook using an estimated channel, and feeds back a codebook index corresponding to a searched weight to a transmitter. In this method, the feedback overhead may be reduced.
In a time-varying channel environment caused by movement of a terminal, it is difficult to perform downlink transmission based on feedback information. In this case, an OL-MIMO based downlink transmission may be relied on. In addition, to maintain acceptable performance in this case, it is required to adapt to the number of spatial streams transmitted by a user. The number of spatial streams or a spatial multiplexing gain depends on not only a Signal-to-Noise Ratio (SNR) typically experienced by a terminal receiver, but also on spatial correlation in a channel. The number of transmitted streams is typically adjusted by applying a precoding matrix at a transmitter. The precoding matrix has a size of N×M, where M is the number of physical antennas and N is the number of spatial streams.
A precoding matrix from a codebook used for CL-MIMO transmission is reused for OL-MIMO transmission. However, since a terminal does not feedback particular information regarding a channel matrix in the OL-MIMO, a method for actually selecting a precoding matrix from a codebook is unclear.
Accordingly, there is a need for an apparatus and a method for codebook design in an environment where a codebook used in the CL-MIMO system is reused in the OL-MIMO system.